Gas turbine engines have been used in the commercial air transport industry for over 30 years. One particular species of engine, termed the "axial flow turbofan", has found wide acceptance and is used as the prime mover in a significant fraction of the current air transport fleet.
The turbofan engine operates by ingesting an axial flow stream of air at the forward end, separating the ingested airstream into an annular fan airflow stream and an inner, coaxial core airflow. The core airflow is further compressed within the engine, subsequently passing through a combustor section wherein liquid fuel is burned to raise the temperature and then through a turbine section wherein the energy of the high temperature, high pressure, high velocity core gas flow is extracted mechanically for driving the upstream air compressor and fan.
The still-hot exhaust gas from the core of the engine is then typically discharged at the downstream end of the engine. The fan airflow and core exhaust gas flow together generate sufficient thrust upon discharge to drive an airframe or the like. It is common practice today to either discharge the annular fan airstream immediately downstream of the fan section, or to contain the fan airstream within an annular flow passage by an outer engine fairing to a downstream point proximate the aft end of the engine.
Historic performance requirements for axial flow turbofan engines used in the air transport industry have specified the provision of high axial thrust at a minimum fuel consumption rate. Due to increased population density and raised environmental consciousness, recent regulations have been enacted requiring a reduction in the level of noise generated by such engines in the vicinity of airports and/or population centers.
While there are several sources of objectionable noise in an axial flow turbofan engine, one particular identified noise generator is the direct discharge of the high velocity, high temperature core gas from the engine. It has been predicted that a 2-3 db drop in overall engine sound generation level may be expected if the discharged core gas and bypass fan air are thoroughly intermixed prior to discharge from the engine. Moreover, the separate discharge of the core gas and bypass fan air in a turbofan engine results in a predicted 1-2 percent loss in overall engine thrust as compared to a fully mixed discharge stream. The advantages of mixing the bypass fan air and core engine exhaust are thus well known in the art.
Prior art mixers have included certain inherent drawbacks which have diminished their attractiveness to both engine and aircraft designers. Such prior systems typically involve a tradeoff between the axial length of the mixing zone and the pressure (and hence energy) loss experienced through the mixing section. By allowing the coaxial streams to mix together without mechanical encouragement, an excessively long axial mixing zone is established. By encouraging mixing in a short axial distance by the use of a convoluted mixing ring or the like, designers have experienced frictional pressure, turbulence, and other internal fluid losses which act to negate the benefits obtained through the mixed discharge of the core and bypass airflows.